1. Field of the Invention
The present invention relates to an arrangement for recovering carrier from received two phase shift keyed signals comprising a band pass filter at the input which is followed by a series connection of a full wave rectifier, a carrier frequency filter and a frequency divider. The two-phase-shift-keyed (2 PSK) signals are phase-shift modulated signals in which the phase position of a carrier only exhibits the values of roughly 0.degree. or 180.degree. .
2. Description of the Prior Art
The transmission of signals by phase shift modulation is well known in the art. The transmission of digital signals usually occurs as a 2 PSK signal or a 4 PSK signal whereby, therefore, the phase position of a carrier is switched between two or four discrete values. When, for example, the digital signal is a binary signal having an approximated equal distribution of logical ones and zeros, then the phase positions of the carrier of 0.degree. and 180.degree. in the 2 PSK signal are distributed equally, so that the amplitude of the carrier is nearly zero. However, it is known that the demodulation of a 2 PSK signal in a disturbed channel can only be optimum with respect to the signal-to-noise ratio or, respectively, the bit error rate of the recovered binary signal when a synchronous demodulator with carrier supplied and proper phase is employed for the demodulation. The necessity of recovering the carrier from the received 2 PSK signal therefore occurs.
FIG. 1 illustrates a known arrangement for carrier recovery for a 2 PSK signal. The band pass filter BP is located at the receiving side input, its center frequency being identical to the carrier frequency and its bandwidth being at least twice the clock frequency or, respectively, the Nyquist frequency of the binary signal. The output signal of the band pass filter is supplied to the signal input of the synchronous demodulator DEM and is also supplied to the input of a full-wave rectifier or squaring element Q. In squaring, the prefiltered 2 PSK signal is multiplied by itself; sum frequencies of all spectral components in the upper and lower sidebands thereby also arise. The components which lie symmetrically relative to the suppressed carrier appear at twice the carrier frequencies; they add up due to the correlation of the upper side band and the lower side band. In a narrow band carrier frequency filter F, these energy components are largely rid of the undesired mixed products which arose during squaring and are output to a following amplitude limiter. After the amplitude limitation, the generation of the carrier occurs in a frequency divider in the division ratio 2:1, this carrier being output to the carrier input of the synchronous demodulator DEM. The demodulation of the 2 PSK input signal occurs in the synchronous demodulator; an amplitude discriminator AE from which a binary signal BS is output is connected to the output of the demodulator by way of a low pass filter TP.
A multiplication of the recovered carrier at the receiving side is undertaken by the signal in the synchronous demodulator; an optimum demodulation of the 2 PSK signal with maximum signal-to-noise ratio only occurs given a proper phase relation of the recovered carrier. Since the phase of the recovered carrier in all carrier recovery circuits for 2 PSK signals can amount to 0.degree. or 180.degree. due to the derivation from the correlation of the signal sidebands or, respectively, due to the 2:1 frequency division, the generated carrier is affected with this uncertainty. In order to avoid this uncertainty in the transmission of 2 PSK signals, a signal in differential binary code wherein the information is transmitted in the signal changes is usually employed as the modulating binary signal.
A relatively high expense for carrier recovery occurs in this known art since, among other things, filters are required for this purpose which must have a high temperature stability and aging stability with respect to the phase stability of the recovered carrier given high selection at the same time.